(a) Technical Field
The present invention relates to a positive electrode material for a secondary battery. More particularly, it relates to a positive electrode material for a secondary battery and a method for manufacturing the same, in which manganese fluorophosphate containing lithium or sodium can be used as an electrode material.
(b) Background Art
With the proliferation of portable electrical and electronic devices, new secondary batteries such as nickel hydrogen batteries, lithium secondary batteries, etc., have been actively developed. Among them, the lithium secondary battery is a battery, in which carbon such as graphite is used as a negative electrode active material, oxide containing lithium is used as a positive electrode active material, and a non-aqueous solvent is used as an electrolyte solution. Lithium having a high ionization tendency can generate a high voltage and is thus used in the development of batteries with high energy density.
Lithium transition metal oxides are widely used as the positive electrode active material, and more than 90% of layered lithium transition metal oxides such as lithium cobalt oxides, lithium nickel oxides, multi-component metal oxides (e.g., cobalt-nickel-manganese ternary system), etc., are used.
However, in the layered lithium transition metal oxides widely used as the positive electrode active material, oxygen separated from a lattice participates in a reaction under abnormal conditions (such as overcharge and high temperature), thus causing abnormal behaviors such as a fire in the battery. In order to overcome the drawbacks of the layered lithium transition metal oxides, extensive research aimed at developing a positive electrode active material having spinel and olivine structures has been pursued.
As a means for solving the problem of the lithium secondary battery, such as a reduction in the safety due to the deterioration of the positive electrode, the use of spinel-type lithium manganese oxides having a three dimensional pathway for lithium ion transport and polyanion-type lithium metal phosphates having an olivine structure, instead of the layered lithium transition metal oxides, has been proposed. However, the use of the spinel-type lithium manganese oxide is limited due to release of lithium during charge and discharge and due to structural instability by Jahn-Teller distortion.
Among the olivine lithium metal phosphates, the use of lithium iron phosphate and lithium manganese phosphate as the positive electrode material has been limited due to low electrical conductivity, but they can be used as the positive electrode material as this problem has been solved by nanosizing of particles, carbon coating, etc.
Recently, fluorophosphates have been reported as the polyanion materials. The fluorophosphate has a chemical formula of A2MPO4F where A is Li or Na and M is a transition metal such as Mn, Fe, Co, Ni, V or a mixture thereof. Since the fluorophosphate theoretically contains two sodium atoms, a theoretical capacity of about two times that of existing lithium metal phosphates is expected.
Moreover, when sodium fluorophosphate (Na2MPO4F, where M is Fe, Co, Ni, V or a mixture thereof) is used as the positive electrode material of the lithium secondary battery, sodium ions are deintercalated during initial charge and lithium ions are intercalated during initial discharge. Then, the intercalation and deintercalation reactions of lithium ions take place during the charge and discharge of the battery. Further, when the sodium fluorophosphate is used as a positive electrode material of a sodium-based battery, the intercalation and deintercalation of sodium ions occurs during the charge and discharge.
U.S. Pat. No. 6,872,492 discloses an example in which sodium fluorophosphates such as NaVPO4F, Na2FePO4F, (Na,Li)2FePO4F, etc., are used as a positive electrode material of a sodium-based battery, but this patent is limited to the sodium battery, not the lithium battery.
Moreover, the structure and the electrochemical properties of sodium iron fluorophosphate (Na2FePO4F) as the positive electrode material for the lithium secondary battery have been disclosed. However, the sodium iron fluorophosphate has a charge/discharge potential of about 3.5 V, which is as low as that of iron-containing olivines. Moreover, the sodium manganese fluorophosphate (Na2MnPO4F) has a potential higher than that of the sodium iron fluorophosphate, but the low electrical conductivity of manganese-containing polyanions leads to electrochemical inactivity.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.